1. Field of the Invention
This invention relates generally to a glow plug to be used for preheating an auxiliary combustion chamber or combustion chamber in a diesel engine, and more particularly to a glow plug for diesel engines comprising a ceramic heater having an quick-heating function and self-saturation properties that permits after glowing for long hours.
2. Description of Prior Art
In general, a diesel engine has poor starting properties at low temperatures. To assist the diesel engine in starting, therefore, a glow plug is usually provided in an auxiliary combustion chamber or combustion chamber to raise the intake air temperature or use as an ignition source the heat generated by applying electricity to the plug. The glow plug is usually of a sheathed heater type constructed by filling a metallic sheath with heat-resistant insulating powder and embedding a heater coil, made of ferrochrome, nickel, etc., in the powder. In addition to this, a ceramic heater type is also known, as disclosed in Japanese patent laid-open No. 41523/1982, which comprises a heating wire, made of tungsten, etc., is embedded in an insulating ceramic material, such as silicon nitride. The ceramic heater type has been widely used in recent years because it has a better heat transfer efficiency and an excellent heat build-up performance since it becomes red hot in a short period of time during heating, compared with the sheathed heater type which involves indirect heating by means of heat-resistant insulating powder and the sheath.
The glow plug of the ceramic heater type, however, has a metallic heating wire, made of tungsten, etc., embedded in the inside of an insulating ceramic material, such as silicon nitride. Because of different coefficients of thermal expansion of both the members, a sharp temperature rise during heating and the repeated use of the heater tend to deteriorate the reliability, including heat resistance, of the ceramic heater. This also results in increased manufacturing costs.
To solve this problem, a ceramic heater construction in which an electrically resistive ceramic material having a coefficient of thermal expansion substantially equal to that of an insulating ceramic material is used as a heating wire has been proposed in Japanese patent laid-open Nos. 9085/1985 and 14784/1985. Both the proposed glow plugs have problems in terms of construction and function, and have not been put into commercial application.
The former, for example, having a construction in which an electrically resistive ceramic material as a heating element is embedded in an insulating ceramic material, exhibits a better heat transfer efficiency than the sheathed heater type. But it has a poor quick-heating function because it is based on indirect heating and involves complex forming and machining. The latter, on the other hand, has a good quick-heating function because its heating element is exposed on the heater surface. The construction of its heating element in which U-shaped members are laminated, with both ends thereof being led to the rear end of the heater, makes the electrode takeoff method extremely complicated, resulting in high manufacturing costs.
In this type of glow plug, there is an increasing market demand in recent years for improved starting performance of the diesel engine, improved durability to withstand the high-temperature service conditions associated with the adoption of turbo-chargers, and the increased use of the after-glowing system in which the glow plug is kept energized for a predetermined period of time after the engine has been started to ensure smooth and adequate combustion inside the engine as exhaust and noise control means. In addition, there is an increasing demand for extending the after-glowing time as long as possible (to approximately ten minutes, for example). That is, even after the diesel engine has been started, it takes much time to warm up the engine in a cold region, for example, because it is chilled down to too low a temperature. In such a non-warmed-up state, engine noise is uncomfortably large during idling. Moreover, the resulting incomplete combustion often causes white smoke, and leading to an engine shutdown in extreme cases. To prevent these troubles, the aforementioned after-glowing and other measures are needed. To achieve longer after-glowing time, it is necessary to substantially improve the heating properties of a heating element while preventing overheating by controlling the power applied to the heating element and to furnish the heater with a self temperature saturation function so as to keep the saturation temperature of the heater below an appropriate temperature limit. Taking these points into account, the development of an inexpensive glow plug which has a quick-heating function and self temperature saturation properties and is excellent in reliability, including heat resistance.